The invention concerns a method for the bridging of operational shut-downs of an ammonia washer of a plant operating according to the semi-direct technique for the removal of ammonia from coking oven gas. The coking oven gas, in connection with the ammonia wash, is subjected to a direct secondary cooling and the ammonia contained in the process water is freed by means of driving off with water vapor (steam) and the addition of alkali.
Various methods are known for the removal of ammonia from the coking oven gas. According to the so-called direct technique, the previously de-tarred gas is led directly into a saturator loaded with an acid, preferably sulfuric acid. The ammonia present in the gas is reacted in the saturator so as to form salt. In practice, however, the direct technique has not previously been capable of being accomplished.
In contrast, with the so-called semi-direct technique, the coking oven gas is initialy cooled in a precooling stage to temperatures from 20 up to 30 degrees C., whereby already part of the ammonia present in the gas is separated out together with the gas water (gas condensate). Thereafter, the pre-cooled and de-tarred coking oven gas is first led into an apparatus likewise loaded with an acid, for removal of the residual ammonia. In the text which follows, this apparatus of interest is always designated as an ammonia wash, for purposes of providing a uniform nomenclature. It is, however, clear to the skilled man of the art that this apparatus can display different construction features, and is frequently also designated in practice as a saturator. The so-called ammonia washer is loaded with sulfuric acid or phosphoric acid so as to provide for the formation of the corresponding ammonium salt. However, this reaction, in which heat is released, leads to increase in temperatures of the oven gas and also to increase in its vapor dew point. Therefore gas must still be subjected to a so-called secondary cooling in connection with the ammonia wash. This secondary cooling can, for example, be performed directly, whereby the coking oven gas is sprayed in one or more stages directly with cooling water led in circulation. Herewith, the accumulating excess circulation water is, in normal manner, treated together with the gas water, which is obtained upon the pre-cooling of the coking oven gas, in an ammonia still. Thereby the free ammonia, by means of the addition of water vapor, and the bound ammonia, by means of addition of alkali, e.g. caustic soda or milk of lime, are liberated; and thereupon subsequently added to the coking oven gas before the ammonia wash. The stripped water running out from the ammonia still is expelled from the process after an appropriate waste water treatment. Compared to the direct techniques mentioned above, the semi-direct techniques have been performed well in practice.
It so happens that the ammonia washer for the removal of ammonia according to the semi-direct technique must occasionally go out of use for purposes of routine maintenance and servicing or for other reasons. In such cases when this happens, the arrangements for the production of the coking oven gas are not cut out of action and, based on the criterion of coking oven gas purity, one can also not avoid having to remove the ammonia from the gas. Accordingly, such operational standstills of the ammonia washer have previously only been bridged by provision of a reserve unit, which upon coming out of line of the normal ammonia washer takes over its function. Obviously, such a reserve unit increases the plant and maintenance costs to a not inconsiderable extent. Moreover, an additional consideration is that the reserve unit is required only during relatively brief "off-line" periods, and is indeed unprofitable from this point of view.